The following comprises a prior art statement in accordance with the guidance and requirements of 37 CFR Sections 1.56, 1.97 and 1.98.
Petroleum coke is generally calcined at high temperature to drive off volatile hydrocarbons and moisture. The calcined product may be used to produce anodes for aluminum manufacture or, in cases where the delayed coke is needle-type, premium coke, the calcined product may be used for the manufacture of graphite electrodes for use in electric arc steel making processes.
Most present day commercial coke calciners are the inclined, rotary kiln type in which the coke is tumbled in a high temperature atmosphere. Such a process requires a large amount of fuel to heat the kiln and there are large heat losses involved in this process. The operation of this type of rotary calciner is described in U.S. Pat. Nos. 4,022,569 and 4,053,365.
Another type of calciner used for calcining delayed petroleum coke is described in U.S. Pat. No. 4,251,323. The coke is calcined in an internally fired, vertical shaft kiln. In this kiln, a moving bed of particulate material flows downwardly through the kiln and is preheated to calcining temperature and substantially devolatilized in the upper section by hot gases moving upwardly through the kiln. Combustion gas such as air, oxygen or oxygen-enriched air is introduced into a combustion zone where combustible components of a recycle gas, as well as some of the particulate material are burned. Combustion gases plus unburned recycle gas heat the downwardly moving bed of material to calcining temperatures in a calcining zone. The upwardly flowing gases preheat the incoming particulate material in the area above the calcining zone. Kiln off gases containing fines and volatile material in the form of vapor and/or mist are subjected to fines removal and scrubbing with the product being a low heat value product gas comprising carbon monoxide, hydrogen and low molecular weight hydrocarbons. A portion of this product gas is injected into the lower end of the kiln as recycle gas. The upwardly moving recycle gas (prior to combustion) cools the calcined material moving downwardly from the combustion zone so that the calcined product leaving the kiln is somewhat cooled and can be readily handled.
In the internally fired vertical shaft kiln of the aforementioned U.S. Pat. No. 4,251,323, combustion gas is injected into the kiln in the combustion zone through a series of water cooled conduits which are positioned transverse to the vertical cylindrical axis of the kiln. While this arrangement of injector tubes provides for good distribution of combustion gas, the presence of transverse bodies in the shaft kiln restricts the downward flow through bridging of the particulate carbonaceous material which is being calcined. Further, there is significant heat loss through absorption of the combustion heat by the coolant in the injector conduits. Such heat loss impairs the efficiency of the kiln since a greater amount of particulate material must be burned to raise the temperature to calcining temperature for the petroleum coke, generally in the range of 2000.degree. F. to 3000.degree. F., preferably above 2400.degree. F. At such temperatures it is necessary to cool the injector conduits since the use of high temperature resistant metals would be impractical from a cost standpoint for this application and the use of mild steel without cooling would quickly result in failure of the injector conduit. Processing at these temperatures is difficult in a shaft kiln unlike the retorting of oil shale in which such high temperatures and such critical control of such temperatures are not necessary.
In order to attain the required very high temperatures, above 2000.degree. F. in a large shaft kiln, it is necessary to provide heat uniformly through the cross-section of the kiln. Attempts to calcine petroleum coke in shaft kilns have been made utilizing externally fired kilns wherein the heat is generated in an annulus outside the coke containing vessel. This is not practical with a large, commercially-sized operation because of heat transfer problems. Attempts to calcine petroleum coke in electrically heated furnaces have also been unsuccessful on a commercial basis. Thus, in order to utilize the advantages of a shaft kiln on a large scale operation, it is necessary to use an internally fired kiln in which combustion gas is injected into an intermediate section of the kiln for combustion of recycle gas and coke to provide the necessary heat. The primary problem with development of an internally fired shaft kiln for calcining petroleum coke is in the provision of a combustion gas distributor which will tolerate the high temperatures necessary. Merely using a metal pipe grid such as is done in retorting of oil shale is not satisfactory, as the metals will not support the load of the coke bed at the temperatures involved.